In short: a submersible pump needs enough horsepower to move your target flow rate against the total dynamic head — the vertical lift to the water table, friction loss in the drop pipe, and the head equivalent of your desired discharge pressure. For a typical home pulling 10 GPM from 150 ft with a 40 psi target, that works out to roughly 1.2 brake horsepower, which rounds up to a standard 1.5 HP motor.
This is a planning aid based on published hydraulic formulas. For well construction, electrical sizing or code compliance, confirm with a licensed well driller or engineer.
How to size a submersible pump
Pump sizing is a two-step calculation. First you find total dynamic head — everything the pump has to push against, expressed as an equivalent height of water. Then you convert that head, together with your required flow, into horsepower.
Horsepower follows directly: water horsepower (the theoretical minimum) equals flow in GPM times TDH in feet, divided by 3960. Brake horsepower — what the motor actually has to supply — divides that by the pump's efficiency, since no pump converts 100% of shaft power into lifted water.
Worked example. Take the calculator's defaults: 10 GPM, a pumping water level of 150 ft, 10 ft of additional lift to the tank, 170 ft of 1¼ in PVC pipe (C = 150), a 40 psi target and 55% pump efficiency. Static lift is 160 ft. Hazen–Williams gives about 3.97 ft of friction loss, and velocity head adds a negligible 0.11 ft. The 40 psi target adds 92.27 ft (40 × 2.3067). Total dynamic head is 160 + 3.97 + 0.11 + 92.27 ≈ 256.3 ft. Water horsepower is (10 × 256.3) ÷ 3960 ≈ 0.647 HP, and brake horsepower is 0.647 ÷ 0.55 ≈ 1.18 HP — which rounds up to a standard 1.5 HP motor once a 10% safety margin is applied.
Depth is not head. A 300 ft well doesn't need twice the pump of a 150 ft well unless the pumping water level is also twice as deep. Many wells sit far shallower than they're drilled, so always size from the actual pumping water level, not the casing depth.
Why lift, pipe size and pressure change the answer
Pumping water level, not static level. As a pump draws water, the level in the casing drops below its resting (static) level — this is the drawdown. Sizing against the static level under-sizes the pump; use the level the well actually reaches under sustained pumping, from a driller's report or a pump test.
Pipe diameter. Friction loss in the Hazen–Williams equation scales with roughly the inverse fifth power of diameter, so a pipe that is one size too small can multiply friction loss several times over. It's usually cheaper to upsize the pipe than to upsize the motor.
Discharge pressure. Every 2.3 ft of head equals about 1 psi, so a target of 60 psi instead of 40 psi adds roughly 46 ft of head on its own — often more than the friction loss and velocity head combined.
Pipe friction loss reference
Head loss in feet per 100 ft of PVC or poly pipe (Hazen–Williams, C = 150), by flow rate and pipe diameter. The calculator interpolates the same formula for your exact pipe length.
| Flow (GPM) | 1 in | 1¼ in | 1½ in | 2 in |
|---|---|---|---|---|
| 5 | 1.92 | 0.65 | 0.27 | 0.07 |
| 10 | 6.93 | 2.34 | 0.96 | 0.24 |
| 15 | 14.68 | 4.95 | 2.04 | 0.50 |
| 20 | 25.01 | 8.44 | 3.47 | 0.86 |
| 25 | 37.81 | 12.76 | 5.25 | 1.29 |
Worked examples
Shallow well, small home
1 HP
8 GPM, 80 ft pumping level, 5 ft added lift, 95 ft of 1 in pipe, 40 psi target, 50% efficiency. TDH ≈ 181.8 ft; brake horsepower ≈ 0.73, rounding up to a 1 HP motor.
Deep well, larger home
5 HP
15 GPM, 300 ft pumping level, 15 ft added lift, 330 ft of 1½ in pipe, 50 psi target, 58% efficiency. TDH ≈ 437.2 ft; brake horsepower ≈ 2.86, needing a 5 HP motor with margin.
Irrigation, high flow
3 HP
25 GPM, 120 ft pumping level, 5 ft added lift, 140 ft of 2 in pipe, 35 psi target, 60% efficiency. TDH ≈ 207.6 ft; brake horsepower ≈ 2.18, rounding up to a 3 HP motor.
What else affects the result
Water density. This calculator assumes fresh water at a specific gravity of 1.0. Brackish or mineral-heavy well water is only slightly denser — see the water density calculator if you need to check an exact figure — and the effect on horsepower is usually under 3%.
NPSH and cavitation. Total dynamic head sizes the motor; it doesn't guarantee the pump avoids cavitation. For deep-set or high-capacity pumps, check net positive suction head against the manufacturer's curve using our NPSH calculator.
Cycling and motor starts. A pump sized right at its duty point but with a small pressure tank will cycle on and off frequently, shortening motor life. Pair the right pump with adequate tank drawdown, and confirm cycle time with our pump cycle time calculator.
Voltage drop. Long cable runs from the wellhead to a deep-set pump can drop voltage enough to affect motor performance; this is a separate electrical calculation from hydraulic sizing.
Frequently asked questions
What size submersible pump do I need for my well?
It depends on your required flow rate and total dynamic head, not on well depth alone. A typical single-family home needs 8–12 GPM and, depending on depth and pipe run, ends up needing somewhere between 1/2 HP and 1.5 HP. Enter your own numbers above rather than relying on a rule of thumb — two wells of the same depth can need very different pumps depending on pipe size and desired pressure.
What is total dynamic head (TDH) and why does it matter for pump sizing?
Total dynamic head is the total resistance a pump must overcome, expressed as an equivalent height of water: the vertical lift from the pumping water level to the discharge point, plus friction loss in the pipe, plus the head needed to reach your target discharge pressure. A pump is rated to deliver a given flow at a given head, so TDH — not depth alone — is what determines the horsepower you need.
How much horsepower does a submersible well pump need?
Horsepower is calculated from flow, total dynamic head and pump efficiency: water horsepower equals flow (GPM) times head (ft) divided by 3960, and brake horsepower equals that figure divided by the pump's efficiency. Most residential submersible pumps run 50–65% efficient, so the motor has to supply noticeably more power than the water alone theoretically needs.
Does pipe size affect the pump size I need?
Yes, substantially. A pipe that is too small for the flow rate creates high friction loss, which adds directly to total dynamic head and can push you up a full horsepower size. Going up one nominal pipe size often cuts friction loss by more than half for the same flow, which is why the calculator treats pipe diameter and material as required inputs, not afterthoughts.
What discharge pressure should I design for?
Most residential pressure switches cycle between about 30 and 50 psi, so 40 psi is a reasonable average to size against. If you know your pressure tank's cut-in and cut-out settings, use the higher cut-out value so the pump is sized for its hardest working pressure, not just the average.
Should I oversize a submersible pump for safety margin?
A small margin, 10–15% above the calculated brake horsepower, is standard practice and is why this calculator rounds up to the next standard motor size. Oversizing by a lot is counterproductive: it causes the pump to short-cycle, wastes energy, and can starve the motor of cooling flow in a narrow well casing.
Sources & references
- Hydraulic Institute. Standards for pump horsepower, head and efficiency (ANSI/HI).
- Williams, G. S. & Hazen, A. (1920). Hydraulic Tables. Also American Water Works Association, Manual M11, Steel Pipe — A Guide for Design and Installation.
- International Bureau of Weights and Measures (BIPM). The International System of Units (SI).
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)